Since high-frequency power amplifiers in transmission units, especially in base stations or radio and television transmitters, participate significantly in the energy consumption of the overall unit and accordingly directly influence operating costs, their level of efficiency is a substantial feature. To achieve large signal bandwidths or to allow a rapid frequency change, the high-frequency bandwidth of the amplifier is also an important criterion. In this context, a wide spacing of the first harmonic is also desirable at the same time, because, with just under one octave bandwidth, it is only possible to achieve required harmonic spacing with a filter connected downstream subject to considerable technical effort.
Many modern transmission standards, for example, MC-GSM, WCDMA, DVB-T/T2, DAB, ATSC, operate with modulations which provide a non-constant envelope. By contrast, with signals with a constant envelope, such as FM, power peaks can occur which are disposed significantly above the average power. In particular, modulations which exploit OFDM provide a high crest factor within the range of approximately 7-10 dB, which corresponds to the ratio of peak envelope power PPEP (PEP=peak envelope power) to average power PAVG. Accordingly, the power amplifiers must be dimensioned for the relatively rarely occurring signal peaks, although they are operated on average with a significantly reduced modulation.
In the case of conventional power amplifiers in AB operating mode, the efficiency increases with increasing modulation and reaches the maximum value with full modulation in the compression range, that is, in the case of the rare signal peaks. Accordingly, a low efficiency of typically 25% is obtained for the average power PAVG in the case of OFDM signals, such as they occur, for example, with DVB-T.
Numerous circuits based on the Doherty principle have been developed in recent years. In this context, the narrow bandwidth of approximately 10-15% in each case, which necessitates an effort-intensive frequency re-tuning in order to cover a relatively larger frequency range, is disadvantageous, see EP 2 698 918 A1. The signal bandwidth is therefore restricted to the relative bandwidth of the amplifier, so that some applications are excluded. The narrow bandwidth is attributable to the frequency behavior of the impedance inverter itself, but also to the delay time and the frequency response of the matching network.
The impedance inverter of a Doherty circuit is typically arranged after the matching network. The bandwidth is disadvantageously influenced by the high impedance level of the impedance inverter and the delay time through the matching network to the transistor. For this reason, it has been attempted in some circuits to arrange the impedance inverter at a very low impedance level close to the transistor and to realize the transformation to the system impedance of typically 50 ohms only after the impedance inverter. Here also, the bandwidth can be increased as a result.
However, single-ended (common mode) transistor amplifiers which generate an unbalanced signal shape and which therefore by their nature provide a high second harmonic, which is also transmitted to the output in the case of broad bandwidths, are conventionally used in this context. A typical single-ended amplifier comprises an input matching network, the transistor and an output matching network, wherein the output matching network can also be arranged after an impedance inverter in order to increase the bandwidth.
Broadband power amplifiers for high powers are typically embodied using push-pull technology (push-pull circuit). This has the advantage of a balanced signal shape, thereby suppressing the even-numbered harmonics on principle. By contrast with a single-ended circuit, a significantly improved harmonic spacing is achieved, especially in the case of the first harmonic, which substantially determines the requirements of a harmonic filter.
FIG. 1 shows such a push-pull amplifier 1. An input signal 2 is supplied to a balun 3 at the input. The balun 3 transforms the signal into a balanced signal and delivers it to a matching network 4. From the matching network 4, the matched signal is supplied to the push-pull transistor 5. An amplified signal is supplied to an output matching network 6 which delivers it to an output balun 7. The output balun 7 transforms the signal back to an unbalanced line system and outputs it as an output signal 8.
Furthermore, the use of an amplifier according to the principle of load modulation, in which the function of the impedance inverter is realized by means of a hybrid coupler, is known from EP 1 609 239 B1. FIG. 2 shows such an amplifier. The amplifier 10 in FIG. 2 contains a first hybrid coupler 13 to which an input signal 11 is supplied. A further input terminal of the hybrid coupler 13 is terminated with a load balancing resistor 12. Outputs of the hybrid coupler 13 are connected to a main amplifier 14 and an auxiliary amplifier 15. Outputs of the amplifiers are connected to matching networks 16, 17, which are connected in turn to a further hybrid coupler 18. This hybrid coupler 18 accordingly fulfils the function of the impedance inverter. One port of the hybrid coupler 18 in this context is terminated with a “length transmission line” 19. An output signal 20 is output at a further output of the hybrid coupler 18.
The invention is therefore based upon one object of providing an amplifier which can cover a broad bandwidth with high efficiency and a wide harmonic spacing.